Extruders are used in many situations in the plastic-processing industry, for example with plastic material in a condition in which it is capable of flow being extruded through a slit nozzle which, as its name implies, is a nozzle construction in which the outlet orifice for the plastic material is in the form of a slit which may be of substantial length. When the orifice is of considerable length, such a nozzle may be referred to as a wide-slit nozzle. This means that, irrespective of the transverse width of the slit, that is to say the dimension thereof perpendicularly to its longitudinal direction, the slit is formed in a long nozzle body for producing extruded articles of very substantial width.
Thus for example a slit nozzle may be used for coating a support or backing material with a plastic material or for producing films, foils, plates or wide shaped members. One form of such a nozzle comprises a nozzle orifice which is defined by two side surfaces of the nozzle body, at least one of the side surfaces being in the form of a flexible lip which is connected to an adjusting means for varying the width of the nozzle orifice, thereby to vary the thickness of the layer of plastic material or film or the like produced by that nozzle. Thus, when the transverse dimension of the nozzle orifice is reduced or increased, the nozzle produces an extrusion of the desired thickness, over the entire length of the nozzle which is therefore the dimension thereof which is transverse with respect to the direction of movement of the extruded article produced by the nozzle. A nozzle orifice may be produced with precisely the required dimensions to give a desired thickness of extruded article. However, this may not guarantee that the extruded article is always produced with the desired thickness. For example, differences in regard to thermal conditions within the molten plastic material may cause the extruded article to be of greater or smaller thickness, when it leaves the nozzle orifice. That in turn may adversely affect the quality of the article produced by the nozzle.
The thicknesses of the extruded articles produced by means of a wide, shaped or round slit nozzle may be measured by suitable equipment for measuring the thicknesses of the articles or the thicknesses of the layers produced by the nozzle. The thickness of the extruded material may be measured continuously over the width thereof. In the case of materials which are extruded as an extruded article or produced in the form of a coating on a backing material. Conventional measuring equipment can indicate the thickness values as measured by the equipment, in optical or graph form, and can thus reproduce the precise position and also the magnitude of any deviation from the desired thickness.
In the slit nozzle referred to above, at least one lip forming one of the side surfaces of the nozzle orifice is flexible and thus bendable. In order to produce movement of the flexible lip, thereby to vary the size of the nozzle orifice, the apparatus in which the nozzle is used may include components which an influence the shape of the lip connected thereto, by way of a change in length which is dependent on temperature, voltage or current. Fitted into the apparatus in the region of the flexible lip are elements which, when the determining parameter to be measured is heat, are in the form of metal pins. The metal pins are caused to expand by the heat, and thus undergo an increase in length, decreasing in length again when they cool down. By virtue of the measurement taken in that way it will be seen that correction of the thickness of the extruded material is required in one region of the apparatus or another so that the operative width of the nozzle orifice can be altered at the appropriate locations by activating the appropriate control component. For that purpose, it is necessary that the flexible lip can be moved towards or away from the other side surface of the nozzle orifice, so as to vary the operative width thereof.
The control elements are distributed at close spacings over the dimension of the nozzle corresponding to its longitudinal extent. These control elements during the working process are always in the same positions relative to the measuring points of the measuring equipment. In that way the measuring system can apply a correction command to the respective element at which adjustment is required.
In the start-up phase of a production process, major corrections may usually be required, which can also cause a delay in starting actual production and which in particular result in the nozzle producing an extruded material or article which is generally useless because it is not of the precisely required dimensions. During actual production, only minor alterations generally have to be made, once the nozzle orifice configuration is appropriately set.
In the event of a break in the production process, for example due to the extruder running out of material, because of a defect in the equipment or as a result of small batch sizes, the extruder may have to be cut back in its output. When that happens, the automatic system providing for regulation in response to temperature is disturbed and even when breaks in operation are of short duration, that system may become unstable.
In addition, in the case of the thermal regulation system described above, the metal pin in question must always be kept at a certain temperature in order to be able to yield to provide for a required opening of the nozzle orifice; the temperature has to be reduced in order to be able to shorten the length of the pin. In order to accelerate that operation of reducing the length of the pin by virtue of a drop in temperature, all the pins always have cooling air flowing around them, to cover the situation that one or other of the pins has to be cooled down. However that results in a considerable level of expenditure in respect of energy which has to be continuously supplied to the equipment and thus wasted.